1. Field of the Invention
The present invention relates to an image signal correcting circuit device for use in an image data processing apparatus, such as a copying machine, a facsimile machine or a video tape recorder, for correcting image signals.
2. Related Background Art
In order to faithfully reproduce original images, such as original documents for a facsimile machine or subjects for a video tape recorder, various types of data processing apparatuses are each provided with a correcting circuit device.
FIG. 1 shows a conventional correcting circuit device. This correcting device is designed to correct a white image signal, e.g., to compensate for shading. In this device, correction is made by multiplying an output of an A/D converter 1 by a correction factor previously stored in a ROM 3 by means of a multiplier 2.
Dark signals, such as fixed pattern noises, are corrected by a correcting circuit device other than the aforementioned one.
FIG. 2 shows such a conventional correcting circuit device of another type. In this correcting circuit device, a dark signal of an image sensor measured by a black box or the like is written in a ROM 13 previously. A dark signal in the output of the sensor is compensated for by subtracting the dark signal stored in the ROM 13 from the image signal obtained by an A/D converter 1 by means of a subtracter 4.
However, in the aforementioned conventional correcting devices, the ROM 3 or 13 must have a large capacity which allows for storage of white or dark correction data, and the size of the overall system is thus increased.
In the conventional correcting device shown in FIG. 1, when it is desired to make all of the bits of the data from the A/D converter 1 effective, both the multiplier and the multiplicand combined in the multiplier 2 must have a number of bits larger than that of the output of the A/D converter 1. Also, the capacity of the ROM 3 is increased. Consequently, the size of the overall system is increased, and high-speed operation of the system is made difficult.
In the conventional correcting device shown in FIG. 2, when it is desired to output 6-bit data, at least 8-bit data is required from both the A/D converter and the ROM 13. Consequently, conversion accuracy and operation speed of the A/D converter 1 deteriorate and the capacity of the ROM 13 is increased.
Furthermore, the correction data previously measured by the black box of the like may not always correspond to the dark signal obtained in an actual operation, and correction accuracy thus deteriorates.